Currently, in a diesel engine which is an internal combustion engine, a ceramics honeycomb structure formed into a tubular shape is commonly used as a filter for collecting soot contained in an exhaust gas (Diesel Particulate Filter: DPF). In such a DPF, a function of collecting soot is given to the honeycomb structure by alternately plugging an opening area of both axial ends in the honeycomb structure (Patent Document 1).
Concretely, as shown in FIG. 2(B), a honeycomb structure 100 made of ceramics is formed to allow transmission of gas through a wall surface 101 that forms the honeycomb, and each thorough-hole 105 (hereinafter, referred to as a “cell”) of the honeycomb structure 100 is plugged in its opening area of either axial end with a plugging material 106, and is open in its other end. Focusing on one axial end in the honeycomb structure 100, when one cell 105a is plugged, another cell 105b neighboring this cell 105a is formed into an open state. In other words, when the honeycomb structure 100 is viewed from the axial end side, plugged cells 105a are arranged in a houndstooth pattern (plugged cell is arranged alternately) (see FIG. 2(A)).
Since the honeycomb structure 100 has such a configuration, a DPF functions as will be described later and an exhaust gas is cleaned.
First, an exhaust gas to be cleaned is supplied from one end in the axial direction of a DPF (left end in FIG. 2(B)). Then, the exhaust gas flows into the DPF from the open cell 105b (inflow cell 105b). Since the other end of the inflow cell 105b is plugged, the exhaust gas enters the neighboring cell 105a (outflow cell) through the wall surface 101. Since the outflow cell 105a is open at its other end (outflow side) in the axial direction of the DPF, the exhaust gas flows out from the other end (right end in FIG. 2(B)) in the axial direction of the outflow cell 105a. That is, in the DPF, since the wall surface 101 partitioning the neighboring cells functions as a filter, and harmful substances (for example, microparticles and the like) contained in the exhaust gas can be captured by the wall surface 101, it is possible to clean the exhaust gas.
Incidentally, a honeycomb structure made of ceramics is generally formed by extrusion molding ceramics, and directly firing the molded body. As shown in FIG. 2(A), in a honeycomb structure having a tubular outer shape and having a square cross-section shape of each cell, the cell near its outer circumferential surface is a cell having an irregular shape with smaller cross-section area than other cells (for example, a cell having a trapezoidal or triangular cross-section, hereinafter referred to as a cell having a small cross-section).
When such a cell having a small cross-section exists, plugging may not be appropriately conducted when each cell of the honeycomb structure is plugged. As described above, each cell of the DPF is plugged in its either one of end parts, but there is a possibility that both of ends are not plugged in a cell having a small cross-section. When such a plugging failure arises, the exhaust gas supplied to the DPF and flown into the cell having a small cross-section directly passes through from one end to the other end of the DPF without passing through the wall face. In other words, the exhaust gas is discharged without being cleaned.
Also, the cell having a small cross-section is likely to collapse at the time of extrusion molding or firing, and when the cell having a small cross-section collapses, a streaky recess will be formed on the outer circumferential surface of the honeycomb structure. When a honeycomb structure with such a streaky recess is used as a DPF, the recess is likely to be an origin of a crack caused by heat shock, and there is a high possibility of damage of the honeycomb structure by heat shock.